The invention pertains to a method and apparatus for reducing batch-to-batch variability in the manufacture of polycarbonate resins and, in particular, to solution blending of polycarbonate batches prior to the recovery of polycarbonate powder by precipitation, concentration and drying.
Polycarbonates (PC) are manufactured by the interfacial reaction of bisphenols, for example, bisphenol-A (BPA) in a methylene chloride and water carrier, with phosgene in a two phase reaction medium including inorganic caustic (e.g. NaOH) and an organic solvent (e.g. dichloromethane).
Variations in the amounts of reactants and the end capping have resulted in batch-to-batch viscosity (e.g. molecular weight) variations in the final polycarbonate product. These variations can result in significant processing difficulties for the end user.
A known method for alleviating viscosity variability has been to combine batches of the finished product, i.e., polycarbonate powder, and dry blend the material in a vessel. The method is somewhat effective to reduce batch-to-batch variability, however, lot-to-lot variability is greater than desired and creates processing problems for the end user. The dry blending expedient is also inefficient. Finally, dry blending is usually effected at the compounding location which is different from the manufacturing location. This reduces productivity and process cost effectiveness both in terms of operational and capital requirements.
It is therefore desirable to decrease the batch-to-batch viscosity variability (e.g. MW) inherent in the batch interfacialpolycarbonate manufacturing process.